Recently, in consideration of environmental issues, a hybrid vehicle that runs on efficient combination of an engine and a motor has been put to practical use. Such a hybrid vehicle has a rechargeable power storage unit mounted thereon, to supply electric power to the motor at the start and/or at the time of acceleration, and to recover kinetic energy of wheels on a downhill and/or at the time of braking.
A structure for a hybrid vehicle as such has been proposed, which is electrically connected to an external power supply such as a commercial power supply and allows charging of the power storage unit mounted on the vehicle. The externally chargeable vehicle mentioned above is capable of running using electric power from the external power supply stored in the power storage unit while keeping the engine stopped, for a relatively short distance when, for example, one commutes or goes shopping, whereby general fuel consumption efficiency can be improved. Such a running mode is sometimes referred to as EV (Electric Vehicle) running mode.
When state of charge (SOC) of the power storage unit attains below a prescribed lower limit after the vehicle runs in the EV running mode, the vehicle enters a common HV (Hybrid Vehicle) running mode, allowing engine operation. In the HV running mode, operation output of the engine is used as driving force for running and, in addition, used for a power generating operation to charge the power storage unit.
As an example of a structure for switching between the EV running mode and the HV running mode, Japanese Patent Laying-Open No. 09-098513 discloses a charge and discharge controller for a hybrid electric vehicle, in which initial battery travel distance is ensured and the ratio of power generation running (hybrid running) is reduced, so as to sufficiently attain the advantage of silence, to prevent decrease of mileage by the internal combustion engine for power generation and to reduce exhaust gas.
Different from the charge and discharge controller for a hybrid electric vehicle disclosed in Japanese Patent Laying-Open No. 09-098513 mentioned above, in a vehicle of which power storage unit can be charged by the external power supply, it is preferred to have the value of state of charge (battery charging rate) in the HV running mode kept at a relatively low value. This is to enable storage of as much electric power as possible from the external power supply at the time of charging with the external power supply after running.
The manner of use of a vehicle, however, differs user by user. Specifically, a user may mostly run relatively short distances while another user may run relatively long distances and, therefore, the degree of executing charging operations using an external power supply (charging frequency) differs user by user.
As a result, in a hybrid vehicle that is not very frequently charged with the external power supply, the value of state of charge of the power storage unit is kept relatively low for a longer period of time. The low charged state kept for a long time is not preferable, considering degradation of the power storage unit.